1. Field of the Invention
The present invention relates to an antenna matching device configured to perform impedance matching between a wide-band antenna and an RF circuit.
2. Description of the Related Art
In accordance with the diversification of communication standards, information communication terminals that are capable of communicating a plurality of types of radio frequency signals are demanded. Frequency bands handled in individual communication standards are different. Accordingly, in a known information communication terminal, individual antennas are disposed in accordance with respective communication standards, that is, in accordance with respective radio-frequency signal bands to be sent and received.
However, in accordance with a reduced size of information communication terminals, there is an increasing demand for the use of a common antenna. Thus, an antenna that is capable of sending and receiving radio-frequency signals in a wide frequency range (broadband) including individual frequency bands handled in multiple communication standards is being used. For example, in some current cellular phones, an antenna that is capable of sending and receiving 800 MHz to 2 GHz band signals is used.
If such an antenna that is capable of sending and receiving wide-band signals (wide-band antenna) is used, it is necessary to perform impedance matching between this antenna and an RF circuit for each of the frequency bands handled in the communication standards. For example, in an antenna device disclosed in Japanese Unexamined Patent Application Publication No. 10-98405, an antenna matching device that can switch between frequency signals by using a switch is disposed between an antenna and an RF circuit.
FIG. 9 is a circuit diagram illustrating a configuration of the related art, such as that disclosed in Japanese Unexamined Patent Application Publication No. 10-98405. A known antenna matching device 10P is connected to an antenna 101 via an antenna terminal PANT. The antenna matching device 10P is connected to an RF circuit 102 via an RF terminal PRF.
A switch 12P is a so-called SPDT switch and switches and connects a first selection terminal or a second selection terminal to a common terminal. The common terminal is connected to the RF terminal PRF. The first selection terminal is directly connected to the antenna terminal PANT only via a transmission line. The path through which the first selection terminal and the antenna terminal PANT are connected to each other serves as a first transmission path 111P. The second selection terminal is connected to the antenna terminal PANT via an inductor 11P. The path through which the second selection terminal and the antenna terminal PANT are connected to each other serves as a second transmission path 112P.
The antenna matching device configured as described above performs control so that the switch 12P is switched in accordance with a frequency to be sent or received. If a communication signal of a first frequency, which is a low frequency, is sent or received, the antenna matching device selects the first selection terminal (first transmission path 111P). If a communication signal of a second frequency, which is a high frequency, is sent or received, the antenna matching device selects the second selection terminal (second transmission path 112P).
With this configuration, the antenna matching device is set so that, when the antenna 101 and the RF circuit 102 are directly connected to each other, the impedance of the antenna 101 and that of the RF circuit 102 will match each other at the first frequency. With this setting, if the antenna 101 and the RF circuit 102 are connected as a result of the switch 12P selecting the first transmission path 111P, it is possible to reduce return loss of a communication signal of the first frequency.
Then, the inductance of the inductor 11P connected to the second selection path 112P is suitably set so that the impedance of the antenna 101 and that of the RF circuit 102 will match each other at the second frequency. With this setting, if the antenna 101 and the RF circuit 102 are connected as a result of the switch 12P selecting the second transmission path 112P, it is possible to reduce return loss of a communication signal of the second frequency.
However, in the above-described configuration of the known antenna matching device, all the transmission lines are connected to the switch. Accordingly, regardless of which frequency band of a communication signal is selected, a communication signal is transmitted via this switch. The switch inevitably causes transmission loss. Thus, regardless of which frequency band of a communication signal is transmitted, transmission loss occurs due to the switch. The transmission loss caused by a switch is greater than that when a communication signal is transmitted via a single passive element, such as an inductor or a capacitor.
Thus, even if the return loss is reduced by the antenna matching device 10P, it is not possible to sufficiently reduce the transmission loss of the antenna matching device 10P, as a whole, for all frequency signals to be transmitted due to the transmission loss of the switch 12P.